Numerous implantable and external medical devices are available or have been proposed for monitoring pathological conditions in patients. Various sensors such as electrodes for sensing cardiac electrogram (EGM) or ECG signals, electrodes for measuring thoracic impedance, pressure sensors, and more have been implemented in conjunction with monitoring algorithms employing implantable or external devices for use in diagnosing heart or lung conditions or in managing therapies delivered to treat such conditions. Early intervention and close monitoring is often key to successful therapy management and best outcomes. However, reliable differentiation between respiratory conditions and cardiac conditions is important in guiding and selecting proper treatment. Because some cardiac conditions can cause respiratory changes, and likewise some pulmonary or airway conditions may cause changes in cardiac function such as heart rate, it can be challenging to determine if an underlying cause of a change in respiration is due to a cardiac condition or a respiration system condition. A need remains for a monitoring apparatus and associated methods for detecting and discriminating cardiac and respiratory conditions.